give cdrom source more priority in apt-get

[Dan Jacobson <jidanni@dman.ddts.net>]

Assuming
$ apt-cache policy twlog
twlog:
  Installed: (none)
  Candidate: 1.3-2
  Version Table:
     1.3-2 0
        500 http://linux.cdpa.nsysu.edu.tw sid/main Packages
        500 cdrom://[Debian GNU/Linux SID _Sid_ - fsn.hu unofficial i386 Binary-4 (20021010)] unstable/main Packages

How can I tell apt-get install that it should try the cdrom first?

Maybe the apt_preferences man page mentions it,

       The final selection method is by origin. This is simply the site
       name of the originating package files. The empty string is  used
       for file URIs.

Is that what I want to use?  If so, how?  Shall I make the cdrom
priority 501?
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